In an electronic musical instrument or game machine, user operation occurs randomly, so the tones that are to be played cannot be anticipated and it has been impossible to generate tones by predicting user operations for sound-expression requests. One requirement for an electronic musical instrument or game machine is that when a request is made for playing a prescribed tone, it must be played immediately. In order to handle such unpredictable requests for immediate expression, the tone data used in electronic musical instruments and game machines has either not been compressed or has been compressed using a compression method whose processing time upon expansion is short, such as, for example, adaptive differential pulse-coded modulation (ADPCM).
In a minidisk (MD), ATRAC (adaptive transform acoustic coding) and ATRAC 2 have been developed as high-efficiency encoding audio compression techniques. These techniques provide high sound quality, predict the coming data (not controlled by user operation) in order to pre-read the data, and are making it possible to realize music playback machines that can generate tones.
ADPCM can compress data to about 1/4, and ATRAC 2 can compress data to about 1/10 to 1/20. Furthermore, the sound obtained by expanding data compressed by ATRAC 2 is closer to the original sound (the pre-compressed sound) than is the sound obtained by expanding data compressed by ADPCM.
However, ATRAC 2 imposes a heavier (about 20-fold) processing burden for compression and expansion than does ADPCM, and for this reason it has been considered unsuitable for electronic musical instruments and game machine, in which user requests for sound generation must produce the desired expression immediately.
Also, in an electronic musical instrument or game machine, requests for the expression of multiple tones are sometimes made simultaneously, which leads to the problem that not all the tones can be generated (expressed) immediately if a compression method requiring heavy processing, such as ATRAC 2, is used.